The invention relates to systems for controlling the operation of a secondary mover, which is driven by a prime mover. The invention is particularly useful in applications wherein the prime mover is "wild", that is to say, no control action may be applied to the prime mover.
One application for the present invention arises in aircraft, wherein hydraulic power is used to move the control ailerons. In most types of aircraft, it is a requirement that an emergency source of hydraulic power be provided which can be used in the event of a failure of the main hydraulic power system. To this end, it is known to employ a prime mover, such as a ram air turbine, for a variable displacement hydraulic pump, the prime mover powering the hydraulic pump in order to provide emergency hydraulic power for the control ailerons, etc. However, since the operation of the prime mover is dependent upon the airspeed of the aircraft, then as the aircraft loses airspeed in an emergency situation, the prime mover loses power and the alternative hydraulic power supply can be lost at a relatively early stage because with a variable displacement pump, a pressure compensator is normally provided which ensures that the pump delivers hydraulic fluid at the flow rate demanded by the system and at a predetermined pressure. Accordingly, if the outlet pressure of the pump falls due to decreasing airspeed, then the pump will automatically try to increase that pressure by increasing the stroke of the pump, resulting in increased pump demanded power leading to stalling of the prime mover. Clearly, this is not acceptable with an emergency hydraulic supply and it is an object of the present invention to provide apparatus which obviates this problem.
Often, in such aircraft, applications, wild AC generators, which are driven by the aircraft engine(s), are provided to power an electric motor pump. It will be appreciated that any variation in engine speed will alter the frequency of the substantially constant AC supply voltage used to energise the motor and hence affect the maximum output power of the system. By way of background, this problem is discussed in general terms below.
A common requirement for a hydraulic pump, such as a variable delivery swash pump, is that the output pressure should be held substantially constant regardless of the flow rate, which may vary widely depending on the load. A swash pump can be converted to a pump of this (constant pressure) type by providing a feed back path by means of which the swash plate or yoke angle is made dependent on the output pressure. This is normally achieved by providing a pressure compensator valve which balances the pump output pressure against a spring. The output from the valve is fed to a piston which controls the angle of the swash plate or yoke. Thus if, for example, the pump output pressure rises, because of the load, the spool of the compensator valve is moved against its spring, providing a path for the high pressure fluid from the pump output to reach the valve output and so move the yoke angle control piston against a yoke restoring spring. This decreases the angle of the swash plate or yoke, so decreasing the flow rate-. This will decrease the pressure to match the new load condition.
Conversely if the pump output pressure falls, because of increased demand, the spool of the compensator is moved by its spring, releasing pressure from the piston allowing the yoke restoring spring to increase the angle of the swash plate or yoke, so increasing the flow rate. This will result in a rise of pressure to match the new load. Provided these load/demand changes are within the capacity of the pump the negative feedback operating will hold the output pressure steady by increasing flow rate on any drop of pressure.
Once the maximum possible yoke angle is reached and the maximum flow rate is achieved, the output pressure will no longer be held constant for any further flow demand but will fall, while the flow rate will remain constant.
It is useful to note also that in the constant pressure region of operation, the torque required to drive the pump and the power are both proportional to the flow rate. The input power is the product of torque/speed and the output power is the product of pressure/flow rate, these are of course equal if a pump efficiency of 100% is assumed.
It has been an implicit assumption so far that the speed at which the pump is driven is constant. The pump requires, of course, a suitable motor to drive it, and an AC electric induction motor is often used for this purpose. The speed of such a motor is not constant. In fact, the torque/speed characteristic of such a motor, driven from a substantially constant voltage supply is such that the speed matches the AC drive frequency for zero load torque, and falls as the torque increases. However, the change of speed is designed to be relatively small for a wide range of torques, and constant speed can therefore be assumed without substantial error. See FIG. 1 of the accompanying drawings.
This relationship only holds good if the torque demanded of the motor is within the torque range of the motor obtainable at sensibly constant speed. Torque demand above this level will enter a region of the motor characteristic exhibiting large changes of speed for small changes of torque. In this region the motor operation is unstable and tends to stall. See FIG. 1 again,
In designing a motor driven pump it is obviously desirable to match the power output requirements of the AG motor to the input power requirements of the pump. For a constant speed application the critical parameter for the motor will be output torque (power being the product of torque and speed) and for the pump, input torque. The pump torque requirement is given by the product of pump displacement and pressure. For a pump operating at a constant pressure the maximum torque requirement occurs at the maximum displacement of the pump.
The input torque characteristics of the pump, over the full range of displacement, is shown in FIG. 2. As the torque is a function of pump displacement and system pressure, for a constant pressure system the characteristic holds good (except for churning losses) over a range of speed. The motor torque output characteristic (see FIG. 1) therefore has to meet, with some margin, the pump input torque requirement (see FIG. 2). This then sizes the motor needed to drive the pump (see FIG. 3). It is however possible, should the hydraulic requirements permit, to reduce the size of the drive motor by the introduction of a soft cut-off pressure compensator control giving the characteristic shown at 4A in FIG. 4 which has a reduced torque demand shown in FIG. 5, optimising both the hydraulic supply and the electrical loading.
This discussion is based on the assumption that the AC supply to the motor is of constant voltage and frequency. If, however, the AC frequency is variable, as in practice it may be, then the motor speed and torque will vary correspondingly (to an acceptable degree of approximation). While the pump outlet pressure will be maintained constant, independent of any speed change, by the pressure feedback control the pump output flow will vary correspondingly with speed provided the motor has sufficient drive torque.
Consider now the effects on the power output of the motor and the relationship to the pump requirements of a constant voltage variable AC supply frequency. It is characteristic of AG induction motors that the speed of the motor is proportional to the supply frequency while the output torque varies inversely with frequency (see FIG. 6) giving essentially constant output power over the frequency range. The effect on the pump being driven at variable speed is to vary the input power requirement. For any given displacement and system pressure the torque required to drive the pump is sensibly the same over a range of speeds (see FIG. 7) therefore as the speed increases the input power requirement increases proportionately with speed.
Torque T=Displacement.times.Pressure+Losses
Therefore, the torque required to drive a given pump at maximum displacement is proportional to pressure and independent of speed, neglecting losses, which while being speed dependent, are small.
It can be seen that a motor pump combination designed to provide a specified flow and system pressure, having a motor with adequate torque to drive the pump at full displacement at the minimum AC supply frequency will, as the AC supply frequency increases, rapidly enter the stall region of the motor characteristic (see FIG. 8).
Since the motor output torque is lowest at the maximum AC supply frequency, one solution would be to size the motor to provide adequate torque at the highest AC supply frequency. This would mean that the hydraulic supply would be grossly in excess of the specified system requirements at the high frequencies and the motor grossly oversized at the low frequencies. Returning, by way of example, to the aircraft hydraulic supply application since Ac supply frequency is tied to engine speed and the high frequencies are only likely to occur during take-off, the maximum power phase of a flight, the penalties of sizing the motor at the high frequencies, namely increased size, weight, cost, electrical power consumption and inefficient hydraulic power generation, are features to be avoided for the major part of a flight regime.
An ideal solution that addresses these penalties would be to limit the pump power requirements as a function of the AC supply frequency or motor speed such that pump input power needs to match the available motor power over the entire frequency range. Since the pump and motor Speeds are the same being mechanically coupled and power is the product of torque and speed, it is necessary to have matched torques if operating in a constant pressure system. This can be achieved by limiting the displacement of the pump as a function of AC supply frequency or unit speed for the required frequency range.
For a system not requiring constant pressure, other control methods may be employed such as a soft cut-off control characteristic indicated at 3A in FIG. 3, and in FIG. 4 where input torque requirements can be limited by a combination of displacement and pressure control associated with AC supply frequency.